A 4-week-old full-term boy is brought to the emergency department due to vomiting. His parents describe the emesis as undigested formula without blood or bile. The vomiting occurs after feeds and has increased in frequency and force over the past 6 days. However, the infant continues to bottle-feed every 1-2 hours. He previously had soft, yellow stools daily but has had no stool for the past 2 days. The patient was born at 39 weeks gestation through meconium-stained amniotic fluid with Apgar scores of 8 and 9. Temperature is 37.2 C (99 F) , blood pressure is 70/30 mm Hg, pulse is 182/min, and respirations are 10/min; pulse oximetry is 98% on room air. Examination shows a thin, sleepy infant with a sunken anterior fontanelle and dry mucous membranes. Cardiac examination reveals tachycardia but no murmurs or gallops. The abdomen is soft, nontender, and nondistended. Which of the following laboratory findings would be expected in this patient?

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11ec25c8_d729_7d44_bc81_d3adcd582a71_MD0003_00 This infant has vomiting with feeds and signs of dehydration (ie, sunken fontanelle, dry mucous membranes) concerning for infantile hypertrophic pyloric stenosis. This condition is most common in first-born boys age 3-5 weeks. The hypertrophied pylorus muscle obstructs the gastric outlet, resulting in nonbilious, projectile emesis. Although many patients have a palpable, olive-shaped abdominal mass, its absence does not exclude the diagnosis. hypertrophied pylorus muscle If diagnosis is delayed, protracted vomiting can result in hypochloremic, hypokalemic metabolic alkalosis due to hydrochloric acid lost in the emesis. Hypovolemia also activates the renin-angiotensin-aldosterone system and potassium is excreted by the kidneys in response to aldosterone. The respiratory system responds to alkalosis with hypoventilation, which results in increased PaCO₂ and a compensatory respiratory acidosis. A thick, elongated pylorus on abdominal ultrasonography is diagnostic, and pyloromyotomy is the treatment of choice. thick, elongated pylorus (Choice A) Primary metabolic acidosis is characterized by decreased pH, PaCO₂, and bicarbonate. Metabolic acidosis can occur when vomiting is accompanied by diarrhea as significant amounts of bicarbonate are lost in the stool. Potassium and chloride are also lost in diarrhea. (Choice B) Primary respiratory acidosis is characterized by decreased pH with elevated PaCO₂ and bicarbonate. This occurs in respiratory depression (eg, narcotic overdose) and hypoventilation syndromes (eg, neuromuscular disease). PaCO₂ retention in pyloric stenosis is compensatory to the primary metabolic alkalosis. (Choice C) A normal acid-base status is characterized by pH of 7.35-7.45, PaCO₂ of 35-45 mm Hg, and bicarbonate of 22-26 mEq/L. Laboratory values may be normal initially in patients with pyloric stenosis but are unlikely in this patient with prolonged vomiting, dehydration, and abnormal vital signs. (Choice E) Hyperkalemia and hyperchloremia are not seen in pyloric stenosis. (Choice F) Primary respiratory alkalosis is characterized by increased pH with decreased PaCO₂ and bicarbonate. This is typically caused by hyperventilation and can be seen in patients with pain or anxiety. Educational objective: Pyloric stenosis presents at age 3-5 weeks with nonbilious, projectile vomiting after each feed. Protracted vomiting produces a hypochloremic, hypokalemic metabolic alkalosis. References: Pyloric stenosis in pediatric surgery: an evidence-based review. (http://www.ncbi.nlm.nih.gov/pubmed/22595707) Electrolyte profile of pediatric patients with hypertrophic pyloric stenosis. (http://www.ncbi.nlm.nih.gov/pubmed/23528507) Formula-feeding and hypertrophic pyloric stenosis: is there an association? A case-control study. (http://www.ncbi.nlm.nih.gov/pubmed/26944185)

A 4-Week-Old Full-Term Boy Is Brought to the Emergency Department